Various techniques have been used for providing power from a photovoltaic array to a power grid. FIG. 1 illustrates one such example in which a photovoltaic (PV) array 11 formed of plural groups 12 of photovoltaic modules, supplies DC voltage to respective combiner circuits 13, each of which combines, in parallel, the outputs from the groups 12 of photovoltaic modules of the array 11. A combiner circuit 13 is also known in the art as a combiner box. The output of the combiner circuits 13 are fed on respective DC lines 14, also called DC feeders, to inverters 17 which convert the DC voltage from the photovoltaic array 11 to an AC voltage. The AC voltage from inverters 17 is stepped up at respective transformers 19 and then supplied, for example, on a medium voltage AC (MVAC) line 21, e.g. at 34 kVAC, to a combining switchgear 23. The combining switchgear combines the AC voltage output from groups of transformers 19 and applies the combined AC voltage to a substation 25 and ultimately to a power grid 27. Among many control functions, the inverters 17 may also perform a maximum power point tracking (MPPT) operation on the photovoltaic modules of array 11 to which they are connected, as known in the art.
FIG. 2 shows a simplified portion of the FIG. 1 system in which a single combiner circuit 13 and inverter 17 are shown. It particularly shows that the voltage output of groups 12 of photovoltaic modules of array 11 is supplied through the combiner circuit 13 on DC lines 14 to inverter 17 inputs. Each of the groups 12 contains a plurality of PV modules connected in series and/or parallel relationship, or may contain one single PV module. Thus, the output power from the array 11 of photovoltaic modules is supplied directly to the inverters 17 through group switches 16 of the combiner circuits 13 and through a disconnect switch 18. As shown in a further simplification in FIG. 2A, this system typically provides a variable DC voltage (e.g. VINV=900V-1500V) on the DC lines 14 input to inverter 17 which is the same voltage (e.g. VPV=VINV) as supplied by the photovoltaic modules of array 11. Inverter 17 controls the variable voltage input thereto by using an MPPT algorithm to control output power from the photovoltaic modules to which it is connected. In the example shown in FIG. 2A, the DC voltage set by inverter 17 at the terminals of a group 12 photovoltaic modules of an array can vary in the range of 900-1500V, but these values will depend on a particular implementation including output characteristics of the photovoltaic modules and design parameters of the system.
FIG. 3 illustrates a portion of another prior art arrangement in which DC-DC converters 29 are provided between groups 12 of photovoltaic modules of array 11 and combiner circuits 13 (only one such combiner circuit 13 and inverter 17 are shown in FIG. 3) and which provides a substantially fixed DC voltage on a DC line 14 (i.e. Feeder #1) input to the inverter 17. FIG. 3A shows a simplified portion of the FIG. 3 arrangement. In this arrangement the inverter 17 sets a fixed DC voltage on lines 14 (e.g. VINV=1500V (fixed) and each DC-DC converter 29 provides MPPT control over the photovoltaic modules within a group 12 to which it is connected. The photovoltaic modules of array 11 supply a variable input voltage (e.g. VPV=900V-1500V to a DC-DC converter 29 which in turn supplies a fixed voltage to the inverter 17 through the combiner circuit 13.
The FIGS. 3, 3A arrangement has advantages over that shown in FIGS. 2, 2A in that since the inverter 17 does not need to perform MPPT and the input voltage to the inverter 17 is fixed, the power density and efficiency of the inverter can be increased. However, the DC-DC converter 29 must also be able to handle the full power which is transferred from the groups 12 of photovoltaic modules of the array 11 to the inverter 17. Thus, although an inverter 17 having improved power density and efficiency can be used in this arrangement, full power DC-DC converters 29 must be provided. As shown in FIG. 3A and in the illustrated example, the output voltage PV of the array can vary in the range of e.g. 900V-1500V, but the output voltage of the DC-DC converters 29, and combiner circuits 13, and at the input to the inverters 17 is fixed at 1500V. As used herein the phrase “fixed” means providing a desired fixed voltage which may have a slight deviation from the desired fixed value caused by inherent equipment and line losses.
With respect to the system illustrated in FIGS. 1, 2, 2A the output from the PV array 11 can vary under control of the inverters 17, in a relatively wide range of output voltages. Using the example noted, the output voltage of an array 11 can be between 900V to 1500V DC. With such a system, and with the inverters 17 providing the MPPT, various inefficiencies are encountered. For example, up to 35% of the capacity of the inverters 17 is not utilized and up to 40% of the capacity (current carrying capacity) of the DC lines between the PV module groups 12 and combiner circuit 13 and the combiner circuit 13 and inverter 17 are not utilized. Stated another way, the DC lines and inverters must be designed to have a rated capacity to handle the maximum power, voltage and current swings which may occur during MPPT operation, as an example.
The FIGS. 3, 3A system provides somewhat of an improvement on the overall system efficiency since the inverters 17 operate on a fixed voltage, e.g., 1500V. Accordingly, the inverter can be fully utilized, while the capacity of the DC lines in the array and to the combiner circuit 13 and from the combiner circuit 13 to the inverter 17 are likewise fully utilized. In other words, the FIGS. 3, 3A system can be more efficient than that of FIGS. 2, 2A system, but at the expense of requiring full power capacity DC-DC converters 29, which must transport the entirety of the power from the PV module groups 12 and array 11 to the inverters 17, and which must also provide MPPT.
A large number of full power rated DC-DC converters may significantly increase the fixed and operational cost of the system and may reduce the overall system reliability. However, the inverters 17 employed in the FIG. 3, 3A system can be cost effective and more efficient than those employed in the FIG. 1, 2, 2A system since they do not have to deal with a fluctuating DC voltage or provide MPPT functionality.
What would be desirable is a system which provides a fixed voltage at the input to inverters 17, but which has lower fixed and operational cost, higher reliability and availability, and lower losses in the DC-DC converters than those employed in the system shown in FIGS. 3, 3A.